Phase change memory coding

ABSTRACT

An integrated circuit phase change memory can be pre-coded by inducing a first resistance state in some cells and the memory, and a second resistance state and some other cells in the memory to represent a data set. The integrated circuit phase change memory is mounted on a substrate after coding the data set. After mounting the integrated circuit phase change memory, the data set is read by sensing the first and second resistance states, and changing cells in the first resistance state to a third resistance state and changing cells in the second resistance state to a fourth resistance state. The first and second resistance states maintain a sensing margin after solder bonding or other thermal cycling process. The third and fourth resistance states are characterized by the ability to cause a transition using higher speed and lower power, suitable for a mission function of a circuit.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/823,508 filed on 25 Jun. 2010, and issued as U.S. Pat. No. 8,634,235on 21 Jan. 2014, which application is incorporated by reference.

PARTIES TO A RESEARCH AGREEMENT

International Business Machines Corporation, a New York corporation andMacronix International Corporation, Ltd., a Taiwan corporation, areparties to a Joint Research Agreement.

BACKGROUND

This invention relates to phase change memory devices.

Phase change based memory materials, such as chalcogenide-basedmaterials and similar materials, can be caused to change phase betweenan amorphous phase and a crystalline phase by application of electricalcurrent at levels suitable for implementation in integrated circuits.The generally amorphous phase is characterized by higher electricalresistivity than the generally crystalline phase, which can be readilysensed to indicate data. These properties have generated interest inusing programmable resistive material to form nonvolatile memorycircuits, which can be read and written with random access.

The change from the amorphous phase to the crystalline phase, referredto as set herein, is generally a lower current operation. Generally, acurrent pulse for a set operation has a magnitude that is not sufficientto melt the active region of a cell, but heats the active region to atransition temperature at which amorphous phase change material tends tochange to a crystalline solid phase. The change from crystalline phaseto amorphous phase, referred to as reset herein, is generally a highercurrent operation, which includes a short high current density pulse tomelt or breakdown the crystalline structure. The reset pulse generallyhas a short duration and quick fall time, so that the phase changematerial cools quickly, quenching the phase change process and allowingat least a portion of the phase change material to stabilize in anamorphous solid phase. The magnitude of the current needed for reset canbe reduced by reducing the size of the phase change material element inthe cell and/or the contact area between electrodes and the phase changematerial, such that higher current densities are achieved with smallabsolute current values through the phase change material element.

One limitation on applications of phase change memory arises from thefact that phase transitions are caused by heat. Thus, heat in anenvironment in which the chip is deployed can cause loss of data, andloss of reliability.

Also, this limitation to use in environments that do not expose thechips to heat creates another limitation on applications of thetechnology. Specifically, the chip may be mounted onto and electricallyconnected to circuitry in a substrate (such as a package substrate, or aprinted circuit board, for example), in a surface mount operation orother mounting process that involves a thermal cycle. For example, thesurface mount operation typically includes a solder reflow procedure,requiring that the assembly (including the chip) be heated to bring thesolder to a temperature about the melting point (or the eutectic point)of the alloy constituting the solder. Other mounting procedures alsoinvolve thermal cycles subjecting the chip to high temperatures. Thismay result in a change in the resistance of the material in these cells,so that the cell is no longer read as programmed.

For this reason, prior art phase change memory chips have not beenavailable that are capable of retaining a data set stored before themounting process. So, board manufacturers are required to store anynecessary code on the chip, after assembly of the circuit board or afterassembly of a system including the circuit board. This makes phasechange memory devices less desirable than other types of non-volatilememory for many uses.

It is desirable to provide a phase change memory chip that can be usedin extreme operating environments. It is desirable to provide a phasechange memory chip that can be coded prior to mounting on a circuitboard, using a process that retains the data during thermal cyclesencountered during board or assembly manufacturing.

SUMMARY

A phase change memory device described herein can hold data throughthermal events, such as a mounting process involving soldering, and inhigh temperature environments.

An integrated circuit comprises an array of single bit, phase changememory cells, including a data set stored therein represented by somememory cells in the array having a first resistance state and by othermemory cells in the array having a second resistance state. The firstresistance state corresponds to a crystalline phase active region havinga first temperature-hardened morphology, and the second resistance statehaving a minimum resistance provided by crystalline phase active regionhaving a second morphology. As the term is used herein, morphologyrefers to the structure and stoichiometry of the memory material, whichcan change locally in response to applied energy at an active region ofa memory element. Thus, the first temperature-hardened morphology andthe second morphology differ by one or more of grain size, stoichiometryof the phase change material, concentration of additives, segregation ofadditives, or other characteristics that contribute to changes inresistance of the active region. The first temperature-hardenedmorphology is characterized by being induced by higher energy currentpulses than the second morphology, being a crystalline phase and byholding a lower resistance than the second morphology under thermalstress that can cause phase transition from the amorphous phase to acrystalline phase in the phase change material of the cell. Also, thetemperature-hardened morphology can have a structure that does not fallin resistance, and does not increase in resistance by more than apredetermined amount, in response to thermal events. The secondmorphology is characterized by being induced at lower energy, being acrystalline phase and maintaining a higher resistance than the firstmorphology under thermal stress that would cause phase transition froman amorphous phase to a crystalline phase in the phase change materialof the cell.

The first morphology can be induced using a set pulse having relativelylong duration and slow fall time, with a magnitude and duration todeliver an energy sufficient to cause the morphology change. The secondmorphology can be induced using a typical set pulse with a fall timeallowing crystalline phase formation, while having an energyinsufficient to cause the lower, first resistance state.

A method for operating a phase change memory is described based oninducing a lower resistance state in some cells in the memory, and ahigher resistance state in some other cells in the memory, where thelower resistance state corresponds with the first morphology and thehigher resistance state corresponds with the second morphology.

A method for manufacturing a circuit including an integrated circuitphase change memory with pre-coding is described, based on coding a dataset in the integrated circuit phase change memory by inducing a lowerresistance state in some cells and the memory, and a higher resistancestate in some other cells in the memory. The process involves mountingthe integrated circuit phase change memory on a substrate after codingthe data set. After mounting the integrated circuit phase change memory,the process involves reading the data set by sensing first and secondresistance states, which correspond to the lower and higher resistancestates after the thermal event of mounting the integrated circuit.

A process can be executed if desired for a particular application, tochange cells in the first resistance state to a third resistance stateand to change cells in the second resistance state to a fourthresistance state. The first and second resistance states maintain asensing margin even after a mounting process that involves solderbonding or other thermal cycling process. The third and fourthresistance states are characterized by the ability to cause a transitionusing higher speed and lower power, suitable for a mission function of acircuit including the integrated circuit phase change memory. In supportof this process, an integrated circuit is described that includes anarray of phase change memory cells with sensing circuits operable infirst and second modes. The first mode is used for sensing data valuesin the array in response to the first and second resistance states usedfor pre-coding. The second mode is used for sensing data values in thearray in response to the third and fourth resistance states which areused during operation of the device. Control circuits and biasingcircuits are coupled to the array, and arranged to execute transitionprocesses for changing from the pre-coded resistance states to theoperational mode resistance states. Processes include reading a data setwith the sensing circuits in the first mode by sensing the first andsecond resistance states, changing cells in the first resistance stateto a third resistance state and changing cells in the second resistancestate to a fourth resistance state so that thereafter, the data set isreadable with the sensing circuits in the second mode. Also, processesexecuted under control of the control circuits and biasing circuitsinclude write processes to write data in the array by inducing the thirdand fourth resistance states in addressed cells, and read processes toread data in the array with the sensing circuits in the second mode.Prior to the transition processes, the integrated circuit can include apre-coded data set represented by some memory cells in the array havingthe first resistance state, and by other memory cells in the arrayhaving a second resistance state.

Phase change materials as formed for use in an integrated circuit phasechange memory have a basis stoichiometry. A process for inducing a lowerresistance state in a memory cell as described herein includes applyinga current pulse to cause a change in the stoichiometry in the activeregion of the cell, to a combination having a lower crystalline phaseresistance than the crystalline phase resistance of the basis material.A pulse used to induce the lower resistance state can have a magnitudebelow the threshold for causing melting of the active region, with aduration long enough to allow stoichiometric changes in the activeregion. For example, where the basis phase change material comprises adielectric-doped Ge_(x)Sb_(y)Te_(z), such as for example Ge₂Sb₂Te₅, alower resistance state can be induced by applying a current pulse causea change in stoichiometry in the active region to a combination havingan increased concentration of antimony Sb. It is found that the lowerresistance state can have a lower resistance than the resistance of asimilar cell having a stoichiometry closer to the basis stoichiometry,such as encountered by a cell which had not undergone the process ofinducing the change. The term “stoichiometry” as used here refers to thequantitative relationship in atomic concentration between two or moresubstances in the phase change material in a volume measurable, forexample, using energy dispersive x-ray spectroscopy (EDX), or equivalenttechniques. Also, as explained above, the grain size of the lowerresistance state can be larger, also contributing to the lowerresistance of the cell.

The higher resistance state used in the pre-coding process can beinduced using a typical set pulse have an energy insufficient to causethe lower, first resistance state. The higher resistance state used inthe pre-coding process can be induced using a reset pulse having a falltime that prevents transition to a crystalline phase and therebypresents formation of the lower, first resistance state.

The third resistance state, into which cells pre-coded into the firstresistance state are changed, can be a higher resistance state inducedby applying a pulse to induce an amorphous phase in the active region ofthe cell. The fourth resistance state into which cells pre-coded intothe second resistance state are changed, can be an intermediateresistance state induced by applying a pulse to induce a crystallinephase in the active region of the cell.

The technology described here enables use of phase change memoryintegrated circuit in systems that rely on non-volatile memory to storeconfiguration data, computer programs and the like, typicallyimplemented using NOR Flash devices, which can be pre-coded. Thus, phasechange memory integrated circuits can be “designed in” to systems,without creating requirements for modifications of manufacturing linesto ensure that an embedded system can be programmed after the system isassembled, and without requiring the added expense of implementing suchprogramming processes.

The temperature-hardened morphology described herein can also be used asa one-time programmable fuse for many integrated circuit applications,such as redundancy coding for memory arrays, chip signatures, chipoption coding and so on.

The technology described here enables use of phase change memoryintegrated circuit in systems that are employed in more extremeenvironments.

Other aspects and advantages of the technology described here are setforth below with reference to the figures, the detailed description andthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing stages in assembly of semiconductor chipsonto a circuit board.

FIGS. 2A and 2B are diagrammatic sketches in sectional view showingstages in a surface mounting procedure.

FIG. 3 is a diagram illustrating a temperature schedule for a surfacemounting procedure.

FIG. 4 is a simplified diagram of a cross-section of a phase changememory cell.

FIG. 5 is a simplified diagram of a cross-section of a phase changememory cell, like that of FIG. 4, in which the active region is in a lowresistance state.

FIG. 6 is a diagram showing resistances of phase change material inmemory cells at an initial state, at a state following a “set”procedure, and at a state following a “reset” procedure according to theprior art.

FIG. 7A is a graph of temperature versus time in an active regionshowing a general comparison for “set” “long set” and “reset” proceduresaccording to an embodiment of the invention.

FIGS. 7B and 7C show representative current pulse shapes for normal setand long set operations, respectively.

FIGS. 7D-7L illustrate alternative pulse shapes for long set pulses.

FIG. 7M is a graph showing resistance drift due to thermal loadingversus set pulse length.

FIGS. 8A-8D are diagrams showing resistance of phase change material inmemory cells at various stages in a cell programming procedure accordingto an embodiment of the invention.

FIGS. 9A and 9B are diagrams showing a relationship of sense amplifiersettings and cell resistances in first and second modes as describedherein.

FIG. 10 is a diagram showing heuristically an effect on cell resistanceof subjecting a chip to a thermal cycle after coding using the long setand set processes described herein.

FIG. 11 is an example of a sense amplifier circuit suitable for use inconnection with a memory array according to an embodiment of theinvention.

FIG. 12 is a simplified block diagram showing an integrated circuitincluding a PCM memory array with transitional and operational modes asdescribed herein.

FIG. 13 is a flow chart showing a pre-mounting write process asdescribed herein.

FIG. 14 is a flow chart showing a transition mode for post-mounting readfollowed by write process as described herein.

FIGS. 15-17 are simplified diagrams of cross-sections of alternativephase change memory cell configurations.

FIG. 18 is a simplified block diagram of a temperature-hardened phasechange memory as described herein.

FIG. 19 is a graph of reset and set state resistance distributions aftera 245° C. 1 hr baking for a test chip.

FIG. 20 is a graph of initial state resistance distributions before andafter the 245° C. 1 hr baking for a test chip.

FIG. 21 is a graph of set state resistance distributions before andafter the 245° C. 1 hr baking for a test chip.

FIG. 22 is a graph of strong set state and initial state resistancedistributions before and after the 245° C. 1 hr baking.

FIG. 23 is a graph of strong set state and reset state resistancedistributions after the 245° C. 1 hr baking

FIG. 24 is a graph of set and reset state resistance distributions ofstrong set cells after a 10M cycling endurance test. The memory windowis larger than one order of magnitude

DETAILED DESCRIPTION

The invention will now be described in further detail by reference tothe drawings, which illustrate alternative specific embodiments andmethods. The drawings are diagrammatic, showing features of theembodiments and their relation to other features and structures, and arenot made to scale. For improved clarity of presentation, in the figuresillustrating various embodiments, elements corresponding to elementsshown in other drawings are not all particularly renumbered, althoughthey are all readily identifiable in all the figures. Also for clarityof presentation certain features are not shown in the figures, where notnecessary for an understanding of the invention. It is to be understoodthat there is no intention to limit the invention to the specificallydisclosed embodiments and methods and that the invention may bepracticed using other features, elements, methods and embodiments.Particular embodiments are described to illustrate the presentinvention, not to limit its scope, which is defined by the claims. Thoseof ordinary skill in the art will recognize a variety of equivalentvariations on the description that follows.

FIG. 1 diagrammatically illustrates stages in an example of constructingan assembly of semiconductor chips on a circuit board or othersubstrate. In this example, the desired selection of chips, such as amicrocontroller unit MCU 12, a phase change memory PCM 14, at least partof which can be used for storing code or other data sets, random accessmemory RAM 16, and an input/output I/O device 18 are “designed in” foran electronic product including a circuit board. The non-volatile memoryin one or more of the selected chips can be programmed by a manufactureras indicated by arrow 11, such as using a program and test system beforeor after packaging of the individual chips. In this example, the PCM 14is programmed using a pre-mounting coding process to yield coded PCM14′. Also, any coding in the other chips can also be carried out at thisstage, to yield a collection of coded chips 12′, 14′, 16′, 18′, adaptedfor a specific use in the field. The coded chips are thereafter mountedas indicated by arrow 13 onto a circuit board (for example, amotherboard) 15 or other substrate. The procedure used to mount thechips on the substrate can require a thermal cycle that heats the chips,during which the memory cells in the coded PCM can be raised to atemperature above a transition temperature for changes in solid phase ofthe phase change memory material. Also, in some embodiments known assystem-on-a-chip SOC devices, a single chip, including a phase changememory array and other circuitry, can be used in place of the fourdevices illustrated in FIG. 1.

As discussed in more detail below, the coded PCM in this example isconfigured to use a first, lower resistance state and a second, higherresistance state to store a data set, where the higher resistance statedoes not transition to the lower resistance state during the thermalcycle encountered during the mounting process.

Optionally, after the thermal cycle encountered during mounting, thecoded PCM can be subjected to a transition process, preferably executedusing on-chip control circuits, to change the cells in the firstresistance state to a third resistance state, and to change cells in thesecond resistance state to a fourth resistance state, to form coded PCM14′, in a post-mounting coded state. Transitions between the third andfourth resistance states used to write data in the memory can be inducedusing set and reset pulses that induce rapid phase change in the activeregion of the cell suitable for operation of the PCM chip for themission function of the electronic product. A process is describedherein for inducing the first resistance state in the pre-mountingprocess that includes applying one or more “long set” pulses which leadto establishing a low resistance state in the cells. A process isdescribed herein for inducing the second resistance state in thepre-mounting process that includes applying in the alternative one ormore set pulses or one or more reset pulses. Although the mountingprocess may cause cells programmed to the second resistance state toundergo some disturbance that is reflected by a lowering of theresistance of the cells, the disturbed cells will maintain a resistancewell above the low resistance range associated with the first resistancestate. For this reason, the data set stored in the PCM chip using thepre-mounting process can be read after mounting. Also, the operatingmode of the chip can be changed after mounting, so that data can be readand written using more efficient set and reset processes.

Alternatively, the chip can be configured to operate in a mode in whichthe cells are maintained in the first and second resistance states,using long set pulses during operation in the field to induce the first,lower resistance state. This can be referred to as a thermally hardenedphase change memory chip, operable in more extreme thermal environments,and can operate in a normal environment without being refreshed.

A representative mounting procedure that includes a thermal process thatcan disturb prior art phase change memory cells is discussed withreference to FIGS. 2A, 2B and 3. Stages in a surface mount operation areillustrated in FIGS. 2A and 2B. In the example shown in these Figs., aleadframe package 300 and a flip-chip package 220 are mounted onto andelectrically connected to bond sites 123, 223 exposed at a package mountsurface of a circuit board 124.

The leadframe package 300 in this example includes a semiconductor die114 mounted active side upward on leads 116. Interconnect pads 113 inthe die are electrically connected to bond sites on bond fingers 115 onthe leads 116 by wire bonds 112. The die, wire bonds, and bond fingersare enclosed in a protective encapsulation 119, constituting a packagebody from which the leads project. The leads 116 have a dogleg shape, sothat the mount feet 117 are situated below the package body, and someclearance is provided between the lower surface of the package body andthe upper surface of the circuit board when the feet 117 of the leadsare at rest on the bond sites 123.

The flip-chip package 220 in this example includes a die 214 mounted ina flip-chip fashion on, and electrically connected to circuitry on, apackage substrate 206. Electrically conductive balls or bumps (typicallymetal, such as gold or solder) 212 are mounted on interconnect pads 213on the die. Bond pads 205 connected to circuitry in the substrate 206are exposed at the die attach surface of the substrate to provide bondsites for attachment of the interconnect balls or bumps. Second-levelinterconnect lands 207 on the opposite side of the substrate areconnected to bond pads 205 by way of the circuitry in the substrate. Anunderfill 219 fills the space between the active side of the die and thedie mount side of the package substrate, to complete the package. Solderballs 216 are mounted on the lands 207 to provide for electricalconnection of the package 220 to bond sites 223 on the circuit board.

The second-level interconnection of the leadframe package 300 and theflip-chip package 220 is made by soldering the leadframe feet 117 andthe second-level interconnect solder balls 216 onto the bond sites 123,223. Typically, prior to mounting the packages, the circuit board isprepared by depositing small amounts of solder or solder paste (forexample by plating or printing) on the bond sites (the solder or solderpaste may optionally be omitted for flip-chip interconnection). Then thepackages are oriented so that the feet 117 and the solder balls 216 arealigned with corresponding bond sites 123, 223 on the circuit board, andthe packages are moved toward the circuit board so that the feet 117 andsolder balls 216 rest on the solder or solder paste 125, 225.

Thereafter the solder or paste (or second-level interconnect solderballs) is heated to reflow the solder and complete the electricalconnection. Typically, where solder or solder paste is provided, feet orballs are wetted by the reflowed solder, so that the solder flows overthe surfaces of the bond sites 123, 223 and onto the lower surfaces ofthe feet and the balls, as indicated at 145 and 265 in FIG. 2B.

Heating to reflow the solder or solder paste, typically by passing theassembly through a reflow oven, requires raising the temperature of theassembly according to a time/temperature schedule suitable for theparticular solder. FIG. 3 shows examples of reflow heating schedules fora conventional solder (broken line) and for a lead-free SnAg solder. Thelead-free solder has a higher melting point and reflows more slowly and,accordingly, the assembly must be brought to a higher temperature andheld there for a longer time for the lead-free solder than for theconventional solder. Particularly, for a typical lead-free solder, thetemperature is raised during a pre-heat phase from about 150° C. toabout 180° C. over a period of about 60 to 120 seconds; and then duringa reflow phase lasting over 30 seconds the temperature is raised morerapidly to a peak temperature approaching about 240° C., and is heldthere for a period of about 10 to 20 seconds or longer and then isallowed to fall; the temperature throughout the reflow phase exceedsabout 220° C. And particularly, for a conventional solder, thetemperature is raised during a pre-heat phase from about 140° C. toabout 170° C. over a period of about 60 to 120 seconds; and then duringa reflow phase lasting over 30 seconds the temperature is raised morerapidly to a peak temperature in approaching about 225° C., and is heldthere for a period of about 5 seconds or longer; the temperaturethroughout the reflow phase exceeds about 200° C.

As noted in background, above, phase change based memory materials,including chalcogenide based materials and other materials, may beemployed in memory cells. One class of chalcogenide based phase changematerials contains as principal components Germanium (Ge), Antimony (Sb)and Tellurium (Te) in various combinations, and materials in this classare referred to as GST materials.

Suitable compositions of phase change material have a basisstoichiometry (as deposited) which is substantially uniform Ge₂Sb₂Te₅.Other stoichiometries of GST include for example, the compoundsGe₂Sb₂Te₅, Ge₁Sb₂Te₄, and Ge₁Sb₄Te₇, and mixtures of these compounds.More generally, basis phase change materials may includeGe(x)Sb(2y)Te(x+3y), where x and y are integers (including 0). Otherbasis phase change materials other than GeSbTe-based materials can alsobe used, including GaSbTe system, which can be described asGa(x)Sb(x+2y)Te(3y), and x, y are integers. Alternatively, the basisphase change material can be selected from a Ag(x)In(y)Sb2Te3 system,where x, y are decimal numbers that can be below 1.

Doped phase change material can be used as well, where the basisstoichiometry of the phase change material is substantially uniform,such as Ge₂Sb₂Te₅, with dielectric doping, such as by including anadditive of about 10 to 15 atomic percent, or more, silicon oxide, otherdielectrics like silicon nitride. See, U.S. patent application entitledDIELECTRIC MESH ISOLATED PHASE CHANGE STRUCTURE FOR PHASE CHANGE MEMORY,application Ser. No. 12/286,874, which is incorporated by reference asif fully set forth herein.

Also, composite doping may be used, such as described in co-pending U.S.patent application Ser. No. 12/729,837, entitled PHASE CHANGE MEMORYHAVING ONE OR MORE NON-CONSTANT DOPING PROFILES, filed 23 Mar. 2010,which is incorporated by reference as if fully set forth herein.

Ge₂Sb₂Te₅ has a melting temperature about 175° C., and otherstoichiometries can have lower melting temperatures. As will beappreciated, this temperature is approached (and may be exceeded) duringthe pre-heat phase of the reflow operation, and then the assembly isheld well above this temperature during the reflow phase. As a result,amorphous phase material may transition to a crystalline phase, therebylosing any data that may have been stored. For this reason, PCM basedmemory has not been coded before mounting in the prior art.

FIG. 4 illustrates a cross-sectional view of a memory cell 400 includinga memory element 416 consisting of a body of phase change material. Thememory cell 400 includes a first electrode 420 extending throughdielectric 430 to contact a bottom surface of the memory element 416,and a second electrode 440 on the memory element 416. The first andsecond electrodes 420, 440 may comprise, for example, TiN or TaN.Alternatively, the first and second electrodes 420, 440 may each be W,WN, TiAlN or TaAlN, or comprise, for further examples, one or moreelements selected from the group consisting of doped-Si, Si, C, Ge, Cr,Ti, W, Mo, Al, Ta, Cu, Pt, Ir, La, Ni, N, O, and Ru and combinationsthereof. In the illustrated embodiment the dielectric 430 comprises SiN.Alternatively, other dielectric materials may be used.

As can be seen in FIG. 4, the relatively narrow width 422 (which in someembodiments is a diameter) of the first electrode 420 results in an areaof contact between the first electrode and the memory element that isless than the area of contact between the memory element 416 and the topelectrode 440. Thus, current is concentrated in the portion of thememory element 416 adjacent the first electrode 420, resulting in theactive region 410 in contact with or near the bottom electrode 420, asshown. The memory element 416 also includes an inactive region 413,outside the active region 410, which is inactive in the sense that itdoes not undergo phase transitions during operation. The memory elementincludes a basis phase change material.

The basis phase change material of memory element 416 in one examplecomprises Ge₂Sb₂Te₅. The basis material can be defined as thecombination of elements selected as the phase change material, anddeposited with a concentration profile that is characteristic of thatmaterial. As additives are combined, the concentrations of the elementsof the basis material do not change relative to one another. Rather, thebasis phase change material is doped with an additive in this examplewhich is silicon dioxide having an additive concentration profile alongthe inter-electrode current path between the bottom and top electrodes420, 440. In this additive concentration profile, the silicon and oxygencomponents of silicon dioxide can have a combined concentration of about15 at % (5 at % Si plus 10 at % O). As mentioned above, other phasechange materials, and other additives may be used as well. The activeregion consists of doped phase change material 411 with the samestoichiometry as deposited of the basis material, and with a solid phaseresulting from handling of the chip during manufacture, where thematerial has a resistance state corresponding the to initial resistancestate in this example.

FIG. 5 illustrates heuristically, the memory cell of FIG. 4, where theactive region 410 consists of doped phase change material 412, having amorphology different from that of the material in the initial state, asa result of applying a “long set pulse.” It has been observed usingtransmission electron microscopy TEM that in an initial state aftermanufacturing of the device, the memory element 416 of phase changematerial, including the active region 410 and the inactive region 413,has a polycrystalline morphology with relatively small grain sizesuniformly distributed above the bottom electrode 420. Also, afterexposing the integrated circuit to one hour of baking at 245° C., it isobserved that a polycrystalline morphology of the initial state does notchange in appearance in a TEM image. After a strong set pulse, asdescribed in more detail below, the morphology changes so that theactive region 410 includes a large grain of phase change material abovethe bottom electrode 420. Also, after exposing the integrated circuit toone hour of baking at 245° C., the large grain above the bottomelectrode is remains visible in a TEM image. After a reset pulse, theactive region comes amorphous above the bottom electrode. Also, afterexposing the integrated circuit to one hour of baking at 245° C., thepreviously amorphous region is crystallized, assuming relatively smallgrain sizes over the bottom electrode.

In the GST based system, it has been observed that the active regionundergoes a shift in stoichiometry as a result of the “long set”operation, that leads to increasing concentration of antimony. Anexplanation of the low resistance achieved using the long set operationcould be that the antimony-rich stoichiometries of GST have relativelylower resistance, as antimony levels increase. Also, the active region410 after a long set pulse can have large grain size, relative to thegrain size that is achieved in the initial state, and in a set stateinduced by a typical set pulse. An explanation of the low resistanceachieved using the long set operation could be that the large grain sizein the active region results in a structure having fewer grainboundaries in the current path during a read, and thus have relativelylower resistance, as grain sizes increase. Also, an explanation of thelow resistance achieved using the long set operation could be that thedielectric doping or other additives and the phase change materialinteract as result of the energy applied by the long set pulse in amanner that reduces the resistance. Any one or all of these explanationscan contribute to the unique characteristics of the morphology achievedusing the long set pulse. This morphology can be termed a“temperature-hardened morphology” because it has a maximum resistancethat does not exceed a predetermined maximum under thermal stress.Testing shows that the thermal processes emulated by baking for 1 hourat 245° C., can be easily tolerated without loss of data using thistechnique.

Similar temperature-hardened morphologies can be expected in other phasechange materials, as the mixing enthalpy of the materials changes withthe thermodynamics of the active region, causing formation of lowerresistance morphologies that are distinguishable from crystal phasemorphologies adopted by memory cells in response to heating, such as bya solder bonding process or other thermal processes.

FIG. 6 illustrates in a log-log plot of bit count versus resistance,representative resistance states (“envelopes”) for test chip configuredas shown in FIG. 5, using a GST material including: an initial state; a“set” state (following a “set” operation); and a “reset” state(following a reset operation). The initial state is the state of thematerial following manufacture of a die including phase change material,in which processes used in manufacture can reach about 400° C. or more,and preceding any “set” or “reset” programming. In this initial state,the phase change material on the chip can assume a polycrystalline phasehaving resistance within the envelope labeled “Initial.” A “set”operation, as noted above, is generally a lower current operation than a“reset” operation, and induced by applying a set pulse having a lengthtypically in a range about 10 microseconds (μsec) or less at a magnitudesufficient to cause transition in the active region of a cell ofamorphous phase material to a crystalline or polycrystalline phase. The“set” state can have a lower resistance than the initial state and, as aresult the resistance envelope for the “set” state is lower. A “reset”operation, as noted above, is generally a higher current operation thana “set” operation, and induced by applying a reset pulse having a lengthtypically in a range tens of nanoseconds at a magnitude sufficient tocause transition in the active region of a cell of crystalline phasematerial to an amorphous phase. As a result of a rapid cut off of areset pulse, the phase change process is quenched and at least a portionof the phase change material stabilizes in the amorphous state. Becausethe “reset” state is more amorphous than the initial state or the “set”state, the resistance envelope for the “reset” state is higher.

An improved coding method, as disclosed herein, includes a “long setpulse,” resulting in a low resistance, “super-set” state. FIG. 7A showspulse shapes, nominally in current magnitude versus time, applied for“set”, “long set” and “reset” operations. A “set” operation is inducedby a pulse having a rising edge (587) on which the temperature of thephase change material increases from ambient to a temperature above thecrystallization transition temperature, nominally reached where thecurrent passes the Tc line in graph, and below the melting temperaturenominally reached where the current passes the Tm line in the graph, andholding the pulse at a level (588) maintaining the material around thattemperature for a time sufficient to establish a “set” state, and havinga falling edge (589) that falls relatively slowly, lowering thetemperature gradually to ambient so that the material settles in acrystalline phase. A “reset” operation entails sharply rising edge (584)to a magnitude that delivers sufficient energy to the active region thatthe temperature of the phase change material exceeds the meltingtemperature Tm, nominally reached where the current passes the Tm linein graph, and holding the pulse at a level (585) maintaining thetemperature at about that temperature for very short time, and having afast falling edge (586) rapidly lowering the temperature to ambient sothat the material does not have time to crystallize before solidifyingin the amorphous phase. A “long set” operation is induced by a pulsehaving a rising edge (581) on which the temperature of the phase changematerial increases from ambient to a temperature above thecrystallization transition temperature, nominally reached where thecurrent passes the Tc line in the graph, and below the meltingtemperature nominally reached where the current passes the Tm line inthe graph, and holding the pulse at a level (582) maintaining thematerial around that temperature for a time sufficient to establish a“set” state, and having a falling edge (583) that falls relativelyslowly, lowering the temperature gradually to ambient so that thematerial settles in a low resistance, temperature-hardened morphologyand crystalline phase, establishing a “super-set” state.

As can be observed from FIG. 7A, the long set pulse deliverssignificantly more energy over a longer interval of time than isencountered in a typical set pulse. In the illustrated example, the peakmagnitude of the long set pulse is below the current level needed toachieve melting temperature Tm. It is expected that embodiments could beimplemented in which the energy delivered during a long set pulse may besufficient to cause temperature in the active region to exceed themelting temperature Tm. However, the slower trailing edge 583 of thepulse ensures that the material in the active region will crystallize asit cools to assume the stable solid phase of the “super-set” state. Theactual pulse shape of the long set pulse can be varied as suits theparticular implementation, and determined empirically.

FIGS. 7B and 7C show a normal set pulse and a long set pulserespectively. In FIG. 7B, a set pulse is illustrated having a pulselength of 3 microseconds, with a maximum current of 600 microamps thatsteps down in six steps of 500 nanoseconds. FIG. 7C shows a long setpulse illustrated having a pulse length of 192 microseconds, with amaximum current of 1760 microamps that steps down in sixteen steps of 12microseconds. A more typical long set pulse would be substantiallyshorter, with lower magnitude. However, the long set pulse will havesubstantially more energy than a standard set pulse, providing energyneeded for kinetic changes in the active region allowing formation ofthe temperature-hardened, long set morphology.

A long set pulse can be characterized as having significantly moreenergy content (integral of power over time), than the energy content ofa set pulse used to establish the higher resistance state. For example,it is known that a long set pulse having at least 10 times the energycontent of a standard set pulse can induce the temperature-hardenedstate. In examples, a long set pulse having more than 100 times theenergy content of a standard set pulse can induce thetemperature-hardened state. In some embodiments, it is expected that along set pulse having at least 2 times the energy content of a standardset pulse can induce the temperature-hardened state, depending on thesize and configuration of the memory cell, and the composition of thephase change material.

FIGS. 7D-7L illustrate a variety of pulse shapes for long set operationswhich can be applied to cause the lower resistance, modified morphologyin the active region of the body of phase change material for the cell.

In FIG. 7D, a single square pulse 700 of relatively long duration andrapid rising and falling edges is applied, with an amplitude above themelting threshold 701 for the phase change material to cause acumulative duration in the high temperature phase sufficient forformation of the strong set morphology in the active region.Representative pulse widths can range from about 0.5 millisecond to over200 milliseconds, depending upon the materials used, the configurationof the memory cell, the number of memory cells in the array, thespecified life of the cell in number of set/reset cycles, and otherfactors.

FIG. 7E shows that the long set operation can be applied as a sequenceof square pulses 702, 703 which cause a cumulative duration in the hightemperature phase sufficient for formation of the strong set morphologyin the active region.

FIG. 7F shows that the long set operation can be applied as a sequenceof square pulses 704, 705, 706 having stepped down magnitudes, whichcause a cumulative duration in the high temperature phase sufficient forformation of the strong set morphology in the active region. The steppeddown magnitudes may prevent formation of interfacial layers andlocalized anomalies in the structure.

FIG. 7G shows that the long set operation can be applied as a singlepulse 707 with a rapid rising edge and a ramp-shaped trailing edge ortail 708 of constant or near constant slope, which causes a cumulativeduration in the high temperature phase sufficient for formation of thestrong set morphology in the active region. The tail 708 of the formingpulse can let the atoms move more gently without a sudden stop (quench),thus preventing the forming of an interface layer in the active region.For pulses having a fast trailing edge which ramps to zero in aninterval shorter than a quench cutoff, the fast trailing edge could beconsidered as a “quench” which results in the material solidifying inthe active region in the amorphous phase. This quench cutoff is about 10nanoseconds in a phase change material based on Ge₂Sb₂Te₅, and will bedifferent for different phase change materials. In the embodiment shownin FIG. 7G, the trailing edge ramps for a time interval significantlygreater than the quench cutoff, e.g. more that twice the quench cutoff,and can be 5 to 10 times longer, or more.

FIG. 7H shows that the long set operation can be applied as a sequenceof pulses 709, 710 having sloped trailing edges with relatively longtails of constant or near constant slope, which cause a cumulativeduration in the high temperature phase sufficient for formation of thestrong set morphology in the active region. For a pulse having a peakcurrent sufficient to cause a temperature in the active region over themelting threshold for a first duration for the phase change material,the sloped trailing edge in which the current magnitude drops over atime interval significantly greater than the quench cutoff, in thisexample can reduce interface formation within the body of phase changematerials.

FIG. 7I shows that the long set operation can be applied as a singlepulse 711 with a rapid rising edge and a ramp-shaped, or sloped,trailing edge or tail 712 of changing slope, going from a relativelyhigh negative slope to a slope close to zero over the length of thetrailing edge, which causes a cumulative duration in the hightemperature phase sufficient for formation of the strong set morphologyin the active region. For a pulse having a peak current sufficient tocause a temperature in the active region over the melting threshold fora first duration for the phase change material, the sloped trailing edgein which the current magnitude drops over a time interval greater thanthe quench cutoff, in this example can reduce interface formation withinthe body of phase change materials.

FIG. 7J shows that the long set operation can be applied as a sequenceof pulses 713, 714 having ramped trailing edges with relatively longtails of changing slope, which cause a cumulative duration in the hightemperature phase sufficient for formation of the strong set morphologyin the active region. Each pulse in the sequence, or only a final pulsein the sequence can be characterized by having a peak current sufficientto cause a temperature in the active region over the melting thresholdfor a first duration for the phase change material, and a slopedtrailing edge in which the current magnitude drops over a time intervalsignificantly greater than the quench cutoff.

FIG. 7K shows that the long set operation can be applied as a sequenceof pulses 715, 716 with amplitudes stepping down, and having slopedtrailing edges with relatively long tails of constant or near constantslope, which cause a cumulative duration in the high temperature phasesufficient for formation of the strong set morphology in the activeregion. Each pulse in the sequence, or only a final pulse in thesequence can be characterized by having a peak current sufficient tocause a temperature in the active region over the melting threshold fora first duration for the phase change material, and a sloped trailingedge in with the current magnitude drops over a time intervalsignificantly greater than the quench cutoff.

FIG. 7L shows that the long set operation can be applied as a singlepulse 717 with a rapid rising edge and a sloped trailing edgeimplemented by a stepping down trailing edge or tail 718, which causes acumulative duration in the high temperature phase sufficient forformation of the strong set morphology in the active region.

FIGS. 7D-7L show a variety of pulse shapes for long set operation. Ofcourse, other pulse formats and pulse sequences can be applied toachieve the result of causing formation of the strong set morphology inthe active region.

FIG. 7M is a graph of a change in resistance (resistance drift) due tobaking for 1 hour at 245° C. for phase change material versus set pulselength. For a given pulse magnitude, a shorter pulse length leaves theactive region in morphology that undergoes negative resistance drift dueto baking. The resistance drift is positive for longer pulse lengths,and saturates when the temperature-hardened morphology is established,at a shift of about 2.5 K ohms for pulse lengths longer than about 60microseconds in this example. For the dielectric doped GST material usedin these examples, a long set pulse between 10 and 100 microsecondsduration, with a maximum current magnitude between 1 milliamp and 100microamps which is stepped to zero in 10 to 20 equal steps, or ramped tozero from the peak near the beginning of the pulse to zero at the end,can move the resistance distribution of the long set cells below 10 Kohms. Longer and/or multiple long set pulses may be needed to tightenthe distribution of the long set cells. For embodiments of thetechnologies described here, a long set pulse can be characterized as acurrent pulse sufficient to cause the active region to assume amorphology in which the resistance drift saturates, due to hightemperature baking of the sort described here.

FIGS. 8A-8D illustrate resistance states in an array operated asdescribed herein. In this example, the processed and annealed cellsprior to programming (FIG. 8A) are in an initial state 692. Coding forsome cells is carried out using a “long set” operation for some cellsand a “soft-set” operation for other cells, to establish “super-set” 698(FIG. 8B) and “soft-set” 694 states (FIG. 8C), respectively, in selectedcells to code a data set. Later the chip can be mounted or subjected toother thermal events, including for example a solder reflow procedure.As noted above, the resistance envelopes of the “super-set” and“soft-set” states are substantially undisturbed by the thermal events.Optionally, the mounted chip may be subjected to a “reset” operation toswitch the cells in a “super-set” state to a “reset” state 696 (FIG.8D), and a “set operation” to switch the cells in the “soft-set” stateto a “set” state (not shown).

In FIG. 8D, the “set” state cells are read as “1” cells and the “reset”state cells are read as “0” cells. Alternatively, the “set” state cellscould be read as “0” cells and the “reset” state cells could be read as“1” cells. A sufficient “window” appears between the “set” and “reset”state resistance envelopes to enable the sense circuitry to discriminatebetween them; that is, the sense circuitry unambiguously reads a givencell as either “0” or “1”.

Data can be coded for using the long-set pulse for one data value, andany one of the initial state, reset state or set state as the other datavalue. The use of the long set pulse to establish a low resistance stateenables the use of crystalline phase morphologies for storing 1 bit ofdata per cell, as distinguished from multibit cells that utilize a resetstate for one of the data values, and more than one crystalline phasemorphology for the more than one other data values.

Phase change memory devices can be designed which operate using the longset mode in the field. In alternatives, after mounting, the integratedcircuit can be configured to change from the long set mode to a standardmode in order to improve the speed of operation. The sense circuitry forembodiments using technology to change the writing processes from thelong set mode to more typical set and reset modes, is capable ofoperating in at least two sense amplify modes: one for sensing after“set” and “long set” operations and prior to solder bonding; and theother for sensing after “set” and “reset” operation following solderbonding. This is illustrated in FIGS. 9A and 9B. The sense amplify levelfollowing “set” and “long set”, before solder bonding (FIG. 9A) mustoperate in the sensing window between the “super-set” state resistanceenvelope 698 and the “soft-set” state resistance envelope 694, asindicated by the arrow marked S.A. 1. The sense amplify level followingsolder bonding, the “set” and “reset” operations (FIG. 9B) must operatein the sensing window between the “set” or “soft-set” state resistanceenvelope 694 and the “reset” state resistance envelope 696, as indicatedby the arrow marked S.A. 2. The initial state resistance envelope 692 isshown here as well, and can be used to represent a data value in someembodiments.

FIG. 10 is a graph showing emulation in which the phase change materialis heated to temperatures and for times corresponding to those of asolder reflow procedure, suggesting that the lower resistance stateenvelope, induced by the long set pulse, is substantially unchanged bythe thermal cycling; and the higher resistance state, such as induced by“soft-set” pulse or “soft-reset” pulse, is slightly broadened asenclosed by broken line 652 in FIG. 10, and shifted lower to achieve arange of resistances with a minimum resistance that exceeds the maximumresistance of the temperature-hardened, lower resistance state.

The pre-coding “soft-set” state may also be implemented using a“soft-reset” operation to implement the second higher resistance statefor the pre-coded device. A “soft-reset” operation involves applying areset pulse to cells that are to adapt to cause a second higherresistance state. The reset pulse results in formation of an amorphousphase active region, before the thermal cycle encountered duringmounting. The amorphous phase active region may shift to a lowerresistance crystalline phase during the thermal cycle, so that theminimum resistance of the higher resistance state is provided by thecrystalline phase morphology that results from the drift in resistancefrom the amorphous phase under thermal stress. However, the resultingsecond resistance state ensures that a cell has a resistance higher thanthe lower resistance state even after the thermal cycle. The minimumresistance of the higher resistance state is provided by a crystallinephase morphology that is different in one or more of grain size,stoichiometry, or dielectric doping structures, than thetemperature-hardened morphology of the lower resistance state.

FIG. 11 is a simplified diagram of a temperature-hardened phase changememory device a memory array 730 and including sense circuitry capableof operating in two sensing modes. The circuit is provided with tworeference current sources 734, 736 switchably coupled to a referencenode 810 and the load circuits (represented by resistor 725) of senseamplifier 750: one reference current source 734 for use in an initialmode in response to engaging switch 720; and the other reference currentsource 736 for use in an operational mode in response to engaging switch721. The phase change memory array 730 is coupled to a sensing node 811and load circuits 726 of the sense amplifier 750, through appropriatedecoding and biasing circuits. Control logic (See, FIG. 12) on the chipcontrols the switching between modes using switches 720 and 721, orother appropriate control circuits. A variety of multi-mode senseamplifier circuits are used in the art for sensing multilevel cells, andsuch circuits are readily adaptable to the present use.

FIG. 12 is a simplified block diagram of an integrated circuit 1010including a memory array 1012 implemented using memory cells having dualmode sensing circuits as described herein, supporting pre-coding of thephase change memory. A word line decoder 1014 is coupled to and inelectrical communication with a plurality of word lines 1016 arrangedalong rows in the memory array 1012. A bit line (column) decoder 1018 isin electrical communication with a plurality of bit lines 1020 arrangedalong columns in the array 1012. Addresses are supplied on bus 1022 toword line decoder and drivers 1014 and bit line decoder 1018. Dual modesense circuitry (sense amplifiers) and data-in structures in block 1024,are coupled to bit line decoder 1018 via data bus 1026. Data is suppliedvia a data-in line 1028 from input/output ports on integrated circuit1010, or from other data sources internal or external to integratedcircuit 1010, to data-in structures in block 1024. Other circuitry 1030may be included on integrated circuit 1010, such as a general purposeprocessor or special purpose application circuitry, or a combination ofmodules providing system-on-a-chip functionality supported by array1012. Data is supplied via a data-out line 1032 from the senseamplifiers in block 1024 to input/output ports on integrated circuit1010, or to other data destinations internal or external to integratedcircuit 1010.

A controller 1034 implemented in this example, using a state machine,controls the application voltage and current from the voltage andcurrent sources in bias circuitry 1036 for the application of biasarrangements for the phase change memory array. The control circuits andbiasing circuits coupled to the array are arranged to execute transitionprocesses to read the data set with the sensing circuits in the firstmode by sensing the first and second resistance states, and to changecells in the first resistance state to a third resistance state and tochange cells in the second resistance state to a fourth resistance stateso that the data set is readable with the sensing circuits in the secondmode, to execute write processes to write data in the array by inducingthe third and fourth resistance states in addressed cells, and toexecute read processes to read data in the array with the sensingcircuits in the second mode by sensing the third and fourth resistancestates. The controller 1034 may be implemented using special-purposelogic circuitry as known in the art. In alternative embodiments,controller 1034 comprises a general-purpose processor, which may beimplemented on the same integrated circuit to execute a computer programto control the operations of the device. In yet other embodiments, acombination of special-purpose logic circuitry and a general-purposeprocessor may be utilized for implementation of controller 1034.

FIG. 13 is a flow chart of a process for pre-coding a data set in aphase change memory chip during manufacturing. The manufacturing processincludes fabricating a wafer including chips that include phase changememory arrays (1300). Commonly, a wafer test sequence is applied usingwafer level testing equipment to characterize the performance of, and todetect failed chips on the wafers (1301). Next, the die on the wafer areseparated and packaged in a single chip or multichip package suitablefor delivery to the customer, such as an original equipment manufactureror other customer who will mount the chip package on a circuit board orother substrate (1302). Commonly, the individual packages, or die, aretested again using test equipment which can be on an assembly linewithin a packaging factory, or elsewhere (1303). If the individual diefail the testing (1304), they may be discarded or otherwise processed(1305). If the individual die pass the testing (1304), then a pre-codingoperation can be executed. The pre-coding operation can be controlledexclusively by on-chip logic in the controller, or controlled by thechip programming equipment coupled with the die, or a combination of useof the on-chip logic and programming equipment can be used. Inalternatives, the pre-coding can be implemented at the wafer test stagebefore packaging of the die. In an example implementation, the programequipment can be programmed to deliver a command interpreted as apre-coding command, followed by address and data information for thedata set to be coded, using the input and output resources on the chiphaving the phase change memory array. The on-chip controller isconfigured to provide a state machine that can then automaticallyexecute a process to address the array, control the biasing circuitry,and control the timing and operation of the memory array, to perform thepre-coding. The pre-coding as described herein includes inducing a lowerresistance state in cells storing logic “0”, such as by applying a longset sequence including one or more long set pulses (1306). In someembodiments, it may be desirable to apply sequence of long set pulses inorder to reduce tail bits in the resistance distribution for cellsstoring the logic “0”. Also, the pre-coding includes inducing a higherresistance state in cells storing a logic “1”, such as by leaving cellsin the initial state, or by applying a set pulse or a reset pulse(1307). It is noted that the assignment of logic “0” and “1” to thefirst and second resistance states respectively could be reversed. Insome embodiments it may be desirable to use “soft” set pulses or “soft”reset pulses in order to induce the second resistance state. The “soft”set pulses have a shorter duration or lower magnitude than standard setpulses. The “soft” reset pulses have a lower magnitude than standardreset pulses. The use of “soft” pulses for inducing the secondresistance state may improve endurance of the phase change memory arraysbeing operated in this manner.

After writing a data set in the phase change memory array using thelower and higher resistance states, the manufacturing process caninclude a step for verifying the pre-coding (1308). The verifying stepinvolves reading the data set using the transition mode setting for thesensing circuitry, and comparing it to the input data set to verifysuccessful programming. The verifying operation can be controlled byon-chip logic in the controller, controlled by the test equipmentcoupled with the die, or controlled by a combination of use of theon-chip logic and test equipment can be used. For example, theprogramming equipment can deliver a command interpreted as a pre-codingverify command, followed by address and data information for the dataset to be verified, using the input and output resources on the chiphaving the phase change memory array. The on-chip controller isconfigured to provide a state machine that can then automaticallyexecute a process to address the array, control the biasing circuitry,and control the timing and operation of the memory array, to perform theread and compare processes needed to verify successful pre-coding. Afterverifying the pre-coding, the chip can be carried forth in amanufacturing process for mounting the chip on a printed circuit boardor other substrate. A similar process can be used to set the state ofphase change memory cell based fuses.

FIG. 14 is a flowchart of a post mounting refresh for one embodiment. Asexplained above, a pre-coded phase change memory in a circuit board orother substrate, in which the pre-coded data is stored using the longset mode, can be refreshed using standard set and reset operations.Thus, in a manufacture process a set of “designed-in” chips for acircuit board is gathered, including a pre-coded phase change memory,and perhaps other pre-coded memory chips. Also, in some embodimentsknown as system-on-a-chip SOC devices, a single chip including a phasechange memory array and other circuitry is provided at this stage. Next,the chips (or chip) are mounted on a circuit board or other substrate,using a process which can involve a thermal cycle. The mounting processcan result in the phase change cells in the lower resistance state toassume the first resistance state (which may be essentially unchangedfrom the lower resistance state), and phase change cells in the higherresistance state to assume the second resistance state (which may have abroadened or lower resistance distribution than the original higherresistance state). For refresh after a solder bonding or other thermalevent, the data set which had been pre-coded in the phase change memoryarray is read using the transition mode settings in the senseamplifiers, sensing the first and second resistance states (1402). Thedata read in this manner can be stored using an on-chip buffer, oralternatively off-chip memory available on the circuit board so that itmay be written back. The data is written back using the operational modefor the phase change memory that is suitable for the mission function ofthe product in which is being applied. Thus, the process includesinducing a third resistance state in cells storing logic “0”, by forexample applying a reset sequence to induce a typical reset state(1403). Also, the process includes inducing a fourth resistance state incells storing logic “1”, by for example applying a set sequence toinduce a typical set state (1404). The transition mode read operationand the write back processes for the operational mode can be controlledby on-chip logic in the controller, controlled by the test equipmentcoupled with the board, or controlled by a combination of use of theon-chip logic and test equipment can be used. For example, the testequipment can deliver a command interpreted in the test mode as atransition mode read command, followed by address and data informationfor the data set to be read, using the input and output resources on thechip having the phase change memory array. The on-chip controller isconfigured to provide a state machine that can then automaticallyexecute a process to address the array, control the biasing circuitry,and control the timing and operation of the memory array, to perform thetransition mode read processes and operational mode write-back neededfor changing the resistance states of the cells storing the data to theoperational mode resistance states.

After the transition mode used to change the pre-mount resistance statesto the operational mode resistance states, a board test can be executed,including a read process to verify accuracy of the code, or test theoperation of the code executed using the operational mode of the sensingcircuitry (1405). It can be determined whether the board passes thetesting at step 1406. If the board fails the testing, it can be returnedfor repair or reprogramming (1407). If the board passes the testing, theproduct can be delivered to a customer or otherwise passed on themanufacturing process to create a finished product (1408).

FIGS. 15-17 illustrate some of the alternative structures for memorycells which can be used in phase change memory devices operated asdescribed herein. The materials described above may be implemented inthe memory cells of FIGS. 15-17, and thus a detailed description ofthese materials is not repeated.

FIG. 15 illustrates a cross-sectional view of a memory cell 1500including a memory element 1516 consisting of a body of phase changematerial along an inter-electrode current path through memory element1516. The active region 1510 can comprise phase change material havingmorphologies as discussed above, corresponding to the resistance statesused to represent data in the cell.

The memory cell 1500 includes a dielectric spacer 1515 separating firstand second electrodes 1520, 1540. Memory element 1516 extends across thedielectric spacer 1515 to contact the first and second electrodes 1520,1540, thereby defining an inter-electrode current path between the firstand second electrodes 1520, 1540 having a path length defined by thewidth 1517 of the dielectric spacer 1515. In operation, as currentpasses between the first and second electrodes 1520, 1540 and throughthe memory element 1516, the active region 1510 heats up more quicklythan the remainder (e.g. inactive region 1513) of the memory element1516.

FIG. 16 illustrates a cross-sectional view of a memory cell 1600including a memory element 1616 consisting of a body of phase changematerial along an inter-electrode current path through memory element1616. The active region 1610 comprises phase change material havingmorphologies as discussed above, corresponding to the resistance statesused to represent data in the cell.

The memory cell 1600 includes a pillar-shaped memory element 1616contacting first and second electrodes 1620, 1640 at top and bottomsurfaces 1622, 1624, respectively. The memory element 1616 has a width1617 substantially the same as that of the first and second electrodes1620, 1640 to define a multi-layer pillar surrounded by dielectric (notshown). In operation, as current passes between the first and secondelectrodes 1620, 1640 and through the memory element 1616, the activeregion 1610 heats up more quickly than the remainder (e.g. inactiveregion 1613) of the memory element.

FIG. 17 illustrates a cross-sectional view of a memory cell 1700including a memory element 1716 consisting of a body of phase changematerial along an inter-electrode current path through memory element1716. The active region 1710 comprises phase change material havingmorphologies as discussed above, corresponding to the resistance statesused to represent data in the cell. The memory cell 1700 includes apore-type memory element 1716 surrounded by dielectric (not shown)contacting first and second electrodes 1720, 1740 at top and bottomsurfaces respectively. The memory element has a width less than that ofthe first and second electrodes, and in operation as current passesbetween the first and second electrodes and through the memory elementthe active region heats up more quickly than the remainder of the memoryelement.

FIG. 18 is a simplified block diagram of an integrated circuit 1810including a memory array 1812 implemented using memory cells having atemperature-hardened phase change memory as described herein. The memorycells store a single bit in which both data values (0 and 1) arerepresented by crystalline phase morphologies, including thetemperature-hardened morphology for the lower resistance state, and ahigher resistance state that includes a minimum resistance that isprovided by a crystalline phase morphology different from thetemperature-hardened morphology. A word line decoder 1814 is coupled toand in electrical communication with a plurality of word lines 1816arranged along rows in the memory array 1812. A bit line (column)decoder 1818 is in electrical communication with a plurality of bitlines 1820 arranged along columns in the array 1812. Addresses aresupplied on bus 1822 to word line decoder and drivers 1814 and bit linedecoder 1818. Sense circuitry (sense amplifiers) and data-in structuresin block 1824, are coupled to bit line decoder 1818 via data bus 1826.Data is supplied via a data-in line 1828 from input/output ports onintegrated circuit 1810, or from other data sources internal or externalto integrated circuit 1810, to data-in structures in block 1824. Othercircuitry 1830 may be included on integrated circuit 1810, such as ageneral purpose processor or special purpose application circuitry, or acombination of modules providing system-on-a-chip functionalitysupported by array 1812. Data is supplied via a data-out line 1832 fromthe sense amplifiers in block 1824 to input/output ports on integratedcircuit 1810, or to other data destinations internal or external tointegrated circuit 1810. As illustrated, phase change memory cells codedusing the temperature-hardened morphology can be used a fuses 1813,coding for redundancy or other uses on an integrated circuit.

A controller 1834 implemented in this example, using a state machine,controls the application voltage and current from the voltage andcurrent sources in bias circuitry 1836 for the application of biasarrangements for the phase change memory array. The control circuits andbiasing circuits coupled to the array are arranged to execute processesto read the data set with the sensing circuits by sensing the first andsecond resistance states, and to execute write processes to write datain the array by inducing the first and second resistance states inaddressed cells using the long set process for the lower resistancestate, and one of the typical or soft set and typical or soft reset, forthe higher resistance state. The controller 1834 may be implementedusing special-purpose logic circuitry as known in the art. Inalternative embodiments, controller 1834 comprises a general-purposeprocessor, which may be implemented on the same integrated circuit toexecute a computer program to control the operations of the device. Inyet other embodiments, a combination of special-purpose logic circuitryand a general-purpose processor may be utilized for implementation ofcontroller 1834.

Evaluations were carried out using vehicle comprising a 0.18 um embeddedPCM test chip set, having a structure like that of FIG. 5. The bottomelectrode diameter in the test chip was from 30 nm to 50 nm in diameter,with reset current between 300 uA and 1 mA. FIG. 6, described above,shows a typical set, reset, and initial state resistance distribution ofa test chip. Set and reset state resistance distributions overlappedafter 245° C. baking (FIG. 19). FIG. 20 shows that the emulatedsoldering process does not change the initial state resistancedistribution of the PCM cells. This may be due to the maximumtemperature of the BEOL process is approximately 400° C., which is muchhigher than the solder bonding temperature. On the other hand, thetemperature experienced by the PCM cells during the set operation issubstantially higher than the maximum BEOL process temperature. Hence,it would be reasonable to expect that the set state should not changeafter a 245° C. bake either. FIG. 21 shows that the set resistance witha set pulse of 20 μs is unchanged after baking. These results suggestthat two differently programmed set (crystalline) states will be able toretain distinct resistance distributions after the solder bondingprocess. The solution for achieving distinct states which can survivethe solder bonding process is to program the “0” state cells with astronger set condition. The “1” state cells can be programmed into thereset state, or for an even larger margin (post solder bonding) the “1”state cells can be un-programmed, i.e. left in the initial state. Asshown in FIG. 22, the initial state distribution before and after bakingremains very stable.

FIG. 7M discussed above shows that the post 245° C./1 hr bakingresistance drift of a set state depends on the set pulse duration.Shorter (<20 μs) set pulses result in a negative resistance drift andlonger set pulses (>20 us) result in a positive resistance drift. Thepositive resistance drifting saturates at 2.5 KΩ when the set pulse islonger than 60 us. The resistance of the strongly set cells is keptbelow 10 KΩ in order to achieve an adequate resistance margin aftersoldering. Typically, 10 us˜100 us of set pulse duration is long enough,depending on magnitude, cell structure and so on, to move the mainresistance distribution of the strongly set cells to below 10 KΩ. Longerand multiple set pulses may be required to tighten the resistancedistribution. The post bake resistance distribution of strongly setcells is below 8 KΩ and this generates a 100 kΩ resistance differencecompared to the initial state cells (FIG. 22). The post 245° C./1 hrbaking memory window between strongly set cells and reset cells is about4 KΩ (FIG. 23). These results confirm the feasibility of thisprogramming method for implementation of a reliable PCMpre-solder-bonding coding scheme.

To investigate the effect of this strong set operation plus the 245° C.bake on PCM reliability, cells programmed and baked with this conditionwere then cycled using standard set and reset programming conditions.FIG. 24 shows that the strongly set cells can be cycled more then 10million times after the emulated soldering baking process withoutdegradation. Transmission Electron Microscopy (TEM) is used to analyzethe physical phenomena behind these operations. Compared to the initialstate, the long set operation generates temperature-hardened morphologyhaving a larger grain region above the bottom electrode which cancontribute to reduction of the resistance of the cell. Low Angle AnnularDark Field Scanning Transmission Electron Microscopy (LAADF-STEM) hasbeen used to analyze the post-baking cells in order to achieve increasedcontrast for examining the grain sizes. Baking the initial state celldoes not change the grain size. This can explain the un-changedresistance of the initial state cell pre and post bake. The strongly setcells have a larger grain size and fewer grain boundaries compared tothe re-crystallized reset state cells after 245° C./1 hr baking This canexplain why the strongly set cells have a lower resistance than there-crystallized reset cells.

What is claimed is:
 1. An integrated circuit, comprising: an array ofphase change memory cells; sensing circuits coupled to the array, thesensing circuits having a first mode and a second mode, the first modesensing data values in the array in response to first and secondresistance states, and the second mode sensing data values in the arrayin response to third and fourth resistance states, wherein the first andsecond resistance states represent a first and second data valuesrespectively, and the third and fourth resistance states represent thefirst and second data values respectively; and control circuits andbiasing circuits coupled to the array arranged to execute transitionprocesses to read a data set with the sensing circuits in the first modeby sensing the first and second resistance states, and to change cellsin the first resistance state to the third resistance state and tochange cells in the second resistance state to the fourth resistancestate so that the data set is readable with the sensing circuits in thesecond mode, to execute write processes to write data in the array byinducing the third and fourth resistance states in addressed cells, andto execute read processes to read data in the array with the sensingcircuits in the second mode by sensing the third and fourth resistancestates, wherein the control circuits and biasing circuits are arrangedto induce the first resistance state by applying a current pulse havinga first magnitude and first duration, and to induce the secondresistance state by applying a current pulse having a second magnitudeand a second duration, and wherein the first duration is greater thanthe second duration, and wherein the first and second magnitudes areless than a magnitude applied to induce an amorphous phase in an activeregion in a cell in the array, and the first magnitude is greater thanthe second magnitude.
 2. The integrated circuit of claim 1, wherein thedata set before the transition processes is represented by some memorycells in the array having the first resistance state and by other memorycells in the array having the second resistance state.
 3. The integratedcircuit of claim 1, wherein the array comprises memory elementscomprising a phase change material having a basis stoichiometry, and thefirst resistance state is characterized by an active region in a cellhaving a stoichiometric combination with a lower crystalline phaseresistance than a crystalline phase resistance of the phase changematerial with the basis stoichiometry.
 4. The integrated circuit ofclaim 1, wherein the array includes basis phase change memory materialcomprising dielectric-doped Ge_(x)Sb_(y)Te_(z).
 5. The integratedcircuit of claim 4, wherein the control circuits and bias circuits arearranged to induce the first resistance state by applying a currentpulse to cause a change in stoichiometry in an active region of thephase change material to stoichiometric combination having an increasedconcentration of antimony.
 6. The integrated circuit of claim 1, whereinthe first resistance state corresponds with a range of resistance havinga maximum resistance less than minimum resistances of ranges ofresistance corresponding to the third and fourth resistance states. 7.The integrated circuit of claim 1, wherein the third resistance state ischaracterized by an active region in a cell having an amorphous phase.8. The integrated circuit of claim 1, wherein the third resistance stateis characterized by an active region in a cell having an amorphousphase, and the fourth resistance state is characterized by an activeregion in a cell having a crystalline phase.
 9. The integrated circuitof claim 1, wherein the control circuits and biasing circuits arearranged to induce the second resistance state including establishing acrystalline phase in an active region of corresponding cells, and toinduce the third resistance state including establishing an amorphousphase in the active region of the corresponding cells.